Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X
Vol. 2(2), 1-7, Feb. (2012)
Res.J.Chem.Sci.
Preparation of Alkali Lead Glass and Glass – Ceramic Compositions as
Electrical Insulators
Shukur Majid M., Kadhim. F. Al-Sultani and Hassan Mohammed N.
College of Materials Engineering, University of Babylon, IRAQ
Available online at: www.isca.in
(Received 19th September 2011, revised 7th October 2011, accepted 26th January 2012)
Abstract
Lead silicate glasses with three composition ratio B1, B2, and B3 of (SiO2, Al2O3, Na2O, K2O, and PbO) were prepared in this
study by conventional melt quenching technique. K2O percentage increases in compositions B2 and B3 to improve physical and
electrical properties. Also glass – ceramic of lead silicate with three composition ratio C1, C2, and C3 of (SiO2, Al2O3, Na2O,
K2O, PbO, and TiO2) were prepared by conventional melt quenching technique as a first stage, and then converted to glass –
ceramic by heat treatments of the parent glass as a first step and render nucleation and crystallization as a second step. 5 wt.%
of TiO2 is used as a nucleation agent in preparing of glass – ceramic. Physical and electrical properties for all prepared
specimens were investigated; we have noted increasing of dielectric strength with increasing of sintering temperatures for glass
samples, where they have maximum value at 600°C. Whereas the glass – ceramics samples have a maximum value at C3
composition. Dissipation factor for glass and glass-ceramic samples have been decreased with increasing of K2O content at all
sintering temperatures. Dielectric constant (εr) for glass samples at low frequencies has a maximum value at B1 composition
and for sintering temperatures of 525 and 550 °C. At high frequencies, it has maximum value at B3 composition and for
sintering temperature of 550°C. Dielectric constant (εr) for glass-ceramic samples at low frequencies has a maximum value at
C1 composition, and at high frequencies it has maximum value at C3 composition.
Keywords: Glass-ceramic, electrical insulators, sintering temperatures.
Introduction
Glass-ceramics are polycrystalline materials formed by the
controlled nucleation and crystallization of special
formulated glasses1. The amorphous glass article is initially
formed and then thermally converted via heat treatment to a
crystalline material, called a “glass-ceramic”. These glassceramic materials go back to their discovery by S.D. Stookey
in the 1950's2. Glass-ceramics are an important class of
materials that have been commercially quite successful,
which is characterized by the lack of porosity and ease of
forming3. Alkali-lead silicate glasses are an important source
of glass ceramics. Such glasses typically contain over 10%
lead oxide PbO. Lead glasses containing 20-30% PbO, 5458% SiO2 and about 14% alkalis are highly insulating and
therefore of great importance in electrical engineering4. The
glass-ceramic of lead glasses possesses increased impact
strength, hardness and thermal shock resistance compared
with conventional non-crystalline glasses5. Glass – ceramic
can also be formed by powder processing methods in which
glass frits are sintered and crystallized. This procedure
somewhat extends to the range of possible glass-ceramics
composition. It allows for surface as well as internal
nucleation6.
The wide variety of applications include electric range tops,
wood stove windows, telescope mirrors, cooking utensils,
dinner ware, building facing materials, radomes, precision
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electronic ports, fluid amplifiers, ink jet printer heads, and
dental prostheses. Also a new high temperature resistant
glass–ceramic coating used for gas turbine engine
components7. The production of glass ceramic type of
alkali–lead silicate glasses has been attempted using various
approaches. These approaches were often simple additions of
different alkali oxides K2O percentages to produce
macroscopically homogeneous ceramic material which has a
specific application in electrical equipments.
Material and Methods
Experimental procedure of glass samples: Three batches
of lead silicate glass composition have been prepared. Table
1 shows the compositions of these batches.
Table-1
Composition of lead- silicate glasses
Oxide
B1 %wt
B2 %wt
B3 %wt
SiO2
63
59
55
Al2O3
1
1
1
Na2O
8
8
8
K 2O
6
10
14
PbO
22
22
22
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Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X
Vol. 2(2), 1-7, Feb. (2012)
Res.J.Chem.Sci
The amount 300 g of batch has been selected of the
compositions (B1, B2, B3) and was calculated on the basis of
its percentage components. Table 1 shows that the
percentage of K2O has been changed increasingly from
composition of batch B1 to composition of batch B3 to
determine the extent of its impact on the melting temperature
and electrical properties.
Potassium and sodium carbonates are used instead of K2O
and Na2O respectively because they act as fluxes better than
their oxides. Weight ratio was calculated by the equation of
combustion of both Na2CO3 and K2CO3, as a liberated gas of
CO2 during the melting process. Each batch according to
percentage in table 1 was prepared by mixing and milling of
raw materials using a ball mill. The main purpose of this step
was to get a homogeneous distribution of oxides within the
mixture, since uniform mixing of the batch materials is very
important to facilitate the melting process and to ensure more
homogeneity in any glass preparation process, where time of
the mixing was 1 hr at 400 rev / min for each batch. Then the
batch powder was heated in a crucible made of silicon
carbide (SiC) by electrical furnace to a temperature about
1200ºC for 2 hours. It may be noted that the powder begins
agglomerate and then softening at 650ºC. Viscous mass of
glass was obtained at 1000°C and elevated temperature to
1200°C was achieved to reduce viscosity of the batch.
Occasionally, stirring of molten glass was done to complete
homogeneity by using graphite stick and then quenched the
molten glass in distilled water bath (melt - quench technique)
to get the cullet of glass, The cullet was dried at 110ºC for a
period of 24 hours. The quenched glass fragment are milled
by electrical mill to a fine powder and then sieved to less
than 45 µm. Then the glass powders are analyzed using x-ray
diffraction to confirm their amorphous nature. All samples
are formulated by cold pressure shaping technique/cold die
compacting (uniaxial pressing), using a hydraulic pressing
device. The prepared powder was placed in steel die of 13
mm diameter and thickness of 4 mm, with applied pressure
of 150 MPa. Every sample treated with polyvinyl alcohol
(PVA) binder was heated at 350°C for a minimum of one
hour to complete the burn-off the binder. The samples were
heat treated at different temperatures 450, 500, 525, 550,
575, 600, 650, and 750ºC, for 1hr with a heating rate of
5ºC/min. The better thermal extent of sintering temperature
for all batches to obtain glass samples without any distortion
was 525, 550, 575, and 600ºC.
Experimental procedure of glass-ceramic samples: Glassceramic in this research was produced in heterogeneous
nucleation. The compositions of the batches are shown in
table 2. All batches are nucleated by TiO2 agent of
concentration 5 wt%. At first stage, we have manufactured
glass samples for compositions C1, C2, and C3 with the
same method which followed in manufacturing of glass
samples for compositions B1, B2, and B3.
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Table-2
Compositions of glass-ceramic samples
Oxide
C1 %wt
C2 %wt
C3 %wt
TiO2
5
5
5
SiO2
60
56
52
Al2O3
1
1
1
Na2O
8
8
8
K 2O
6
10
14
PbO
20
20
20
The glass samples should be adjusted and controlled by heat
treatment schedule in order to promote the process of
crystallization and conversion to the glass-ceramic. This
regime is first included the nucleation step, which was done
for all the glass samples by conventional heating in an
electric furnace. Crystallization step is followed by the
conventional heating process. In such two-stages of heattreatments, the low-and-high temperatures are designated as
the nucleation and the crystallization treatments respectively.
The glass samples were heated to the nucleation temperature
of (350ºC, 3 hours ramp) in an electric furnace, and held for a
certain period of time (3 hours). After such nucleation, the
nucleated glass disk was crystallized by other stage, in which
the nucleated glass samples were heated above to the
crystallization temperature 550 ºC and 3 hour ramp) in order
to grow the formed nuclei as crystals and hence to convert
the glass into glass – ceramic material.
Results and Discussion
Result of X-Ray Diffraction Analysis: Glass: Figure 1
shows the XRD patterns for lead silicate glasses of the
batches B1, B2 and B3 respectively. This figure indicates
that the prepared glasses were quenched successfully and an
amorphous structures was achieved, where the glass powder
(or samples) are basically composed of amorphous glassy
phase and there are no crystalline phases. Such observation
led to suggestion that these glasses are composed of
assemblages of very small crystals, termed crystallite. It is
well established that measurable broadening of X- ray
diffraction peaks occurs for particle sizes or grain sizes
smaller than 0.1 micron. So, the broadening increases
linearly with decreasing particle size8.
B3
B2
B1
Figure-1
XRD Patterns of glass samples
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Research Journal of Chemical Sciences ______
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Vol. 2(2), 1-7, Feb. (2012)
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Figure 2 displays XRD patterns for lead silicate glass
glassceramic of the batches C1, C2 and C3 respectively. These
batches are treated in two stage as 350°C for 3 hr. and 550°C
for 3 hr. This figure shows that the heat treatment which
used was successfully crystallized into glass-ceramic
ceramic with its
characteristic XRD peaks. The crystal phase identification
and the crystallographic planes corresponding to the 2θ value
for all the peaks were performed using the xx-ray diffraction
file index. XRD chart of batch C1 shows that the glass
glassceramic samples is mainly composed of crystalline phases of
Alumosite PbSiO3 and plumalsite Pb4Al2(SiO3)7. Therefore,
the samples
amples are consist of two phases crystalline and
amorphous, which characteristics of glass ceramic structure.
The main reason for appearing the crystalline phases
(Alumosite and plumalsite) is the modifier adding K2O to the
primary lead glass, in which, a bond in the network is broken
and the relatively mobile potassium ion becomes a part of the
structure. With increase in the amount of modifier, the
average number of oxygen-silicon
silicon bonds forming bridges
between silicon atoms decreases and any further increase
incre
in
modifier would reduce the length of the chain. So, the
principal effect of a modifier is to lower the melting and
working temperature by decreasing the viscosity. An excess
of modifier as in C3 sample can make the structural units in
the melt sufficiently
iently simple and mobile that crystallization
occurs in preference to the formation of a glass9.
`
C2
C1
C3
Figure-2
XRD patterns of glass ceramic samples C1, C2, and C3
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Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X
Vol. 2(2), 1-7, Feb. (2012)
Res.J.Chem.Sci
Results and discussion of electrical properties: Dielectric
strength: Figure 3 shows the effect of sintering temperatures
on dielectric strength for glass samples, where the dielectric
strength increases with raising of sintering temperature,
because the raising in sintering temperature causes
decreasing in porosity, since porosity is affected inversely on
the dielectric strength (i.e. decreasing in the porosity values
will increase the dielectric strength of glass samples). It will
be interesting to know that the increasing in porosity will
generate interior electrical fields and causes loss in dielectric
strength10.
Figure 4 shows the effect of compositions on dielectric
strength for glass- ceramic samples. Glass ceramic of the
composition C1 has a lowest value of dielectric strength,
whereas composition of the glass ceramic batches C2 and C3
have a higher values of dielectric strength. As a matter of
fact, K2O percentage is considered the main reason for
increasing in the values of dielectric strength from batch C1
to batches C2 and C3, in which K2O is acted as an active flux
and in result the increasing in its percentage in the batch will
reduce the porosity and in turn the dielectric strength is
increasing.
Dielectric constant (εr): Glass: The dielectric constant of a
glass results from electronic, ionic, dipole orientation and
space charge contributions to the polarizability. It increases
with the increase of total polarization rate. Only electronic
(Pe) and ionic (Pi) polarization are taken into account when
high frequency is applied. The electronic and ionic
polarization are related to ion radius and its atomic weight,
respectively. The larger radius of the ion results in the higher
(Pe), whereas higher atomic weight results in the lower (Pi).
The dielectric constant of sample B3 is totally attributed to
contributed by K+ polarization created by presence of K+ ion.
Since K+ ion has a large atomic radius 1.33 A°, and their
percentage 14 % in the batch B3 should gain high electronic
polarization11,12. Figure 5 shows the relationship between
frequency (ƒ) and every of dielectric constant (εr) and
dissipation factor for prepared glass samples B1, B2, and B3
respectively. These batches are sintered at different
temperatures and the frequency applied was in the range (40
Hz – 5 MHz).
It may be noted from previous figure 5, at low frequencies,
the maximum values of dielectric constant were at sintering
temperature 525ºC, and the minimum values at sintering
temperature 600ºC for all compositions B1, B2, and B3. This
behavior is related to the porosity of the sample, where it is
high at low sintering temperature, and then causes increasing
in polarization which results from interstitial shipments at
low frequencies13.
Figure 6 make clear the relationship of polarization and
dielectric constant (εr) with frequency. It may be noted from
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that figure the polarization is high at low frequencies due to
space charge contributions resulting from high porosity7,14.
In figure (5) the samples which have the same sintering
temperature, show the same behavior in which the dielectric
constant (at low frequencies) be as follow:
(εr B1 ) >( εr B2 ) > (εr B3 )
It has been observed that the batch composition B3 has lower
value of dielectric constant. This batch as mentioned before
has 14% K2O in its composition and acts as a flux when the
sample is heat treated, so the result becomes lower porosity
and hence acquires lower dielectric constant. On the other
hand, the batch B1, in which K2O % equal to 6 %, shows
dielectric constant (εr) for sintered samples (B1-525, B1-550)
a maximum value at low frequencies result of high porosity
which increases the effect of space charge polarization
contribution. Whereas, the dielectric constant (εr) decreases
with increasing the frequency to reach minimum value at
frequency range (1-2) MHz. The reasons of that are: i. the
absence of space charge polarization effect at high
frequencies, ii. the effect of ionic and electronic polarization
on dielectric constant (εr) for composition B1 was less than
its effect on compositions B2 and B3, because there is low
percentage of K2O in composition B1 relative to B2 and B3
compositions.
It may be noted that the batch B2, in which K2O % equal to
10% and the sintered samples (B2-525 and B2-550) has a
dielectric constant (εr) less than that of the samples with
composition B1 at low frequencies, because low effect of
space charge polarization occurred (due to low porosity
content) which results from the acquiring large K2O % on
comparison with composition B1. Also, it was observed from
figure 5 that the dielectric constant (εr) value decreases with
increasing the frequency to reach minimum values at
frequency range (2.5-3) MHz. The reasons of that are: i. The
absence of space charge polarization effect at high
frequencies, ii. the low effect of ionic and electronic
polarization on dielectric constant (εr) (due to low percentage
of K2O) relative to composition B3. The dielectric constant
(εr) value at high frequency for B2 samples is higher than
that of B1 samples due to increase in K2O percentage, and inturn stepping up the effect of electronic polarization at high
frequency, because the (Pe) and (Pi) are related to ion radius
and atomic weight. In other words, K+ has large in size and
contributes high (Pe), as we previously explained.
The of batch B3, in which K2O % equal to 14 % and the
sintered samples (B3-525 , B3-550) has the lowest value of
dielectric constant (εr) at low frequencies in comparison with
compositions B1 and B2. The reason is the low effect of
space charge polarization at low frequencies (due to low
porosity), as a result to more active flux of K2O % content in
the samples of batch B3 than that of the samples of batches
B1 and B2.
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Vol. 2(2), 1-7, Feb. (2012)
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14
8
Dielectric strength kV / mm
Dielectric strength kV / mm
12
10
8
6
B1
4
B2
B3
2
0
525
550
575
7
6
5
4
3
2
1
0
600
C1(6%)
Temperature °C
C2(10%)
C3(14%)
K2O %
Figure-3
Effect of sintering temperatures on dielectric strength
for glass samples
Figure-4
Effect of compositions on dielectric strength
for glass-ceramic samples
10
300
B 1-525
B 1-550
8
B 1-575
7
B 1-600
B 1-525
250
B 1-550
Dissipation Factor
Dielec tric cons tant ( εr)
9
6
5
4
3
B 1-575
200
B 1-600
150
100
2
50
1
0
0
500000
1000000
1500000
2000000
2500000
3000000
3500000
4000000
4500000
0
5000000
0
500000
1000000
1500000
F req uenc y H z
3500000
4000000
4500000
5000000
4000
B 2-550
8
B 2-525
3500
B 2-575
7
Dissipation Factor
Dielectric constant ( εr )
3000000
4500
B 2-525
9
B 2-600
6
5
4
3
B 2-550
B 2-575
3000
B 2-600
2500
2000
1500
2
1000
1
500
0
0
0
500000
1000000
1500000
2000000
2500000
3000000
3500000
4000000
4500000
5000000
0
500000
1000000
1500000
Frequency Hz
2000000
2500000
3000000
3500000
4000000
4500000
5000000
Frequency Hz
1.8
10
9
1.6
B 3-525
B 3-525
B 3-550
8
1.4
B 3-575
7
Dissipation Factor
Dielectric constant (εr )
2500000
Frequency Hz
10
`
2000000
B 3-600
6
5
4
3
B 3-550
B 3-575
1.2
B 3-600
1
0.8
0.6
0.4
2
0.2
1
0
0
0
500000
1000000
1500000
2000000
2500000
3000000
3500000
Frequency Hz
4000000
4500000
5000000
0
500000
1000000
1500000
2000000
2500000
3000000
3500000
4000000
4500000
5000000
Frequency
Figure-5
The relationship between frequency and both of dielectric strength and dissipation factor for glass samples B1, B2, and B3
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Figure-6
Frequency effects on dielectric material [7]
10
9
700
C 2-( 350 ºС 3 hr + 550 ºС 3 hr )
600
7
C 1-( 350 ºС 3 hr + 550 ºС 3 hr )
C 3-( 350 ºС 3 hr + 550 ºС 3 hr )
6
Dissipation Factor
D ielectric C onstant ( εr )
C 1-( 350 ºС 3 hr + 550 ºС 3 hr )
8
5
4
3
2
C 2-( 350 ºС 3 hr + 550 ºС 3 hr )
500
C 3-( 350 ºС 3 hr + 550 ºС 3 hr )
400
300
200
1
100
0
0
500000
1000000 1500000 2000000 2500000 3000000 3500000 4000000 4500000 5000000
Frequency Hz
0
0
500000
1000000 1500000 2000000 2500000 3000000 3500000 4000000 4500000 5000000
Frequency Hz
Figure-7
Relationship between frequency (ƒ) and both of dielectric constant f(εr) and dissipation factor for glass - ceramic samples
at different compositions and have the same heat treatment
Glass-Ceramic: Figure 7 shows the effect of frequency on
dielectric constant (εr) for glass-ceramic samples of three
compositions C1, C2 and C3.
It may be noted from figure 7, (at low frequency) that: εr
C1> εr C2>εr C3. The main reason of that be high
contribution of space charge polarization Ps(C1)>
Ps(C2)>Ps(C3). This is a result of high porosity for C1
composition relative to C2 and C3 compositions. Where, the
porosity is gradually raised from C1 composition to C2 and
finally reached to more porosity of C3 composition. This
behavior is related to the densification rates of glass ceramic
compositions in which the increment of K2O% will cause
increasing in densification and results of decreasing porosity.
Also we shall explain the behavior of dielectric constant of
these compositions at high frequency as below: for samples
in composition C1 in which K2O=6%, the dielectric constant
(εr) value is decreased by increasing frequency to minimum
value at (3 MHz). The reasons of that are: i. the absence of
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space charge polarization effect at high frequencies, ii. the
low effect of ionic and electronic polarization on dielectric
constant (εr) for composition C1 relative to compositions of
C2 and C3, because there is low percentage of K2O % in
composition C1 in comparison with C2 and C3
compositions. For samples in composition C2 in which K2O
=10%, the dielectric constant (εr) value is decreased by
increasing frequency to reach stable value at high frequency
due to the effect of high ionic and electronic polarization at
high frequency. For samples in composition C3 in which
K2O =14%, the dielectric constant (εr) value is decreased by
increasing frequency to reach stable value at high frequency
due to the effect of high ionic and electronic polarization at
high frequency. As a matter of fact that the composition C3
has stable dielectric constant (εr) value at high frequency
more than dielectric constant (εr) value at high frequency for
composition C2.
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Dissipation factor (Dielectric loss): As a result of
comparison figure 6 with figures 5 and 7 which describe the
relationship between dissipation factor and dielectric
constant with frequency. It will be interesting to know that
there are many effects of polarization on dielectric constant
and dissipation factor. The figure 6 shows the maximum
value of polarization and dielectric constant at low
frequencies, where all types of polarizations in material
(space charge, dipolar, ionic, and electronic) exist at low
frequencies.
It may be noted from above figures that the dielectric
constant at sintering temperatures
(525 and 550)ºС
decreases with increasing frequency to become minimum
value at low frequencies, because of losses space charge
polarization effect (as we previously explained), therefore the
dissipation factor has a maximum value at losses of space
charge polarization.
Energy losses in dielectrics result from three primary
processes: ion migration losses, ion vibration and
deformation losses, and electron polarization losses. The
major factor affecting the usage of ceramic materials is the
ion migration losses15. Modifying cations have a significant
effect on the dielectric loss of the glasses, for example, the
order of mobility of alkali ions Li+ > Na+ > K+ , therefore
the dielectric losses of glass contains Li+ is more than glass
of Na+ content and consequently the glass of K+ content has
less dielectric losses11,15.
while glass–ceramic sample of C3 has a maximum value of
dielectric constant at high frequencies.
References
1.
Berezhnoi A.I., Glass-ceramic and photositalls, New
York Plenum Press, (1970)
2.
McMillan P.W., Glass-ceramics, London: Academic
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Allen M.A., High Temperature Oxides, part iv,
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Wolfram H. and George B., Glass-Ceramic
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Morsi M.M., Crystallization of Lithium Disilicate
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Guy A.J., Essentials of Materials Science, McGrawHill, (1976)
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Charles A.H., Hand book of Ceramics Glasses and
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Madhumita G., Sengupta P., Kuldeep S., Rakesh K.,
Shrikhande V.K. Ferreira J.M.F. and Kothiyal G.P.,
Crystallization behaviour of Li2O–ZnO–SiO2 glass–
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10.
Hill G.H. and Morse C.T., The Effect of Porosity on
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11.
Zhongjian W., Yichen H., Hongkai L. and Fang Y.,
Dielectric properties and crystalline characteristics of
borosilicate glasses, www.elsevier.com (2007)
12.
Pampuch R., Ceramic Materials: an Introduction to
their Properties, Elsevier Scientific Pub. Comp.
Amsterdam, (1976)
13.
Barsoum M.W., Fundamental of Ceramics, The Mc
Graw-Hill companies Inc., (1997)
14
Kingery W.D., Bawen H.K. and Uhlmann D.R.,
Introduction to Ceramics, 2nd Edition Wiley New
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15.
Sharaf El-Deen L.M. and Elkholy M.M., The
dielectric polarizability of amorphous Cu2O–Bi2O3
glasses,
http://www.complexity.org.au/vol09/elkhol01/
(2002)
Conclusion
Lead silicate glasses are characterized by good dielectric
properties. Also, The increasing of K2O percentage in this
type of glasses will increasing the dielectric properties, and
reduces the dissipation factor for all samples. The percentage
of K2O effects on crystallization of this glass, where the
maximum percentage may cause the crystallization and the
optimum percentage was 6%. The increasing of K2O%
reduce the melting point and facility of glass production, and
the temperature of 1200°C is enough to convert the raw
materials of three compositions B1, B2, and B3 to
amorphous structures. Adding 5% of the crystallization agent
TiO2 in this type of glass may cause a crystallization in the
glassy matrix. The optimum range to sinter the glass samples
was (525- 600)°C, and the optimum heat treatment to
deduce a crystallization in glassy phase was (350 for 3 hr and
550 for 3 hr). The decreasing in porosity will increase the
dielectric strength. Glass sample B3 has a maximum
dielectric strength at sintering temperature 600°C. Whereas
in glass-ceramic samples C3 has a maximum dielectric
strength. The maximum value of dielectric constant at low
frequencies for sintered glass sample was 525°C, while the
maximum value of dielectric constant at high frequencies
was 575°C. For glass–ceramic sample of C1 composition has
a maximum value of dielectric constant at low frequencies,
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